1. Field of the Invention
The present invention is related to a flexible transistor having an oxide semiconductor layer.
A transistor of the present invention can be used for devices such as a liquid crystal display, an organic light emitting display, a micro capsule type electrophoresis-type display, a photo-write type cholesteric liquid crystal type display, a TwistingBall type display, a toner type display, a movable film type display and an RFID (RadioFrequencyIdentification) sensor.
2. Description of the Related Art
Generally the transistor which used an amorphous silicon semi-conductor or a polysilicon semi-conductor as a driving transistor of electron device has been used. However, film formation temperature equal to or more than 200 degrees Celsius is required for manufacture of an amorphous silicon semi-conductor and polysilicon semi-conductor of high quality. Therefore, it was difficult to realize a flexible transistor with the use of an inexpensive plastic substrate.
The transistor which used organic semiconductor in semiconductor layer is studied energetically to realize a flexible transistor with the use of an inexpensive plastic substrate. Because organic semiconductor can be made at cold temperature, organic semiconductor can be formed on a plastic substrate. However, mobility of organic semiconductor is extremely low. In addition, degradation of organic semiconductor is easy to occur. Therefore, practical use of organic semiconductor has not yet come.
In such a situation, for example, a transistor having a semiconductor layer made of In—Ga—Zn—O, which showed high mobility even if it was made at room temperature, has been proposed. (K. Nomuraetal. Nature, 432, 488 (2004))
According to this literature, by using In—Ga—Zn—O as a semiconductor layer, a transistor having an excellent semi-conductor of about 10 cm2/Vs mobility on a polyethylene terephthalate substrate could be made in room temperature.
However, when In—Ga—Zn—O is made by sputter method, the semiconductor characteristic is extremely sensitively influenced by oxygen concentration during formation of film.
For example, when In—Ga—Zn—O is deposited under a condition of 1% oxygen flow rate (argon flow rate 100%), it becomes an electric conductor of which conductivity as a film is more than 1 (S/cm). On the other hand, when In—Ga—Zn—O is layered under a condition of 50% oxygen flow rate, it becomes a insulating material of which conductivity as a film is less than 1*10−13 (S/cm).
Oxygen concentration at a film formation surface of a semiconductor greatly influences the number of oxygen deficiency to be included in a semiconductor film. Therefore, in order to make a semiconductor characteristic stable, it is extremely important to control oxygen concentration at a film formation surface of a semi-conductor film.
When inorganic materials such as glass are used as a substrate, deoxidation from a substrate does not almost occur during deposition of a semiconductor. Therefore, control of oxygen concentration during deposition of semiconductor is easy.
However, when a plastic substrate is used as a substrate, deoxidation from a plastic substrate occurs. Therefore, control of oxygen concentration at a surface of semi-conductor film formation becomes very difficult.
In addition, water vapor contained in a plastic substrate scatters to a layer of a semi-conductor when time passes in a case of the transistor that a layer of a semi-conductor of In—Ga—Zn—O is formed on a plastic substrate. Therefore, mobility and an ON/OFF ratio of a transistor fall.
In the present invention, oxygen and water vapor from a plastic substrate are shielded by a gas barrier layer, and they do not go to an oxide semiconductor layer. And in the present invention, a flexible structure with a transistor of which characteristic is stable and of which mobility and on/off ratio are kept to be high is provided.